The invention relates to a method for the methanol synthesis from hydrogen, carbon monoxide and carbon dioxide under pressure, comprising at least one synthesis stage of the type specified in the introductory part of claim 1; as well as to a system for such a synthesis.
A number of systems and methods for catalytic methanol syntheses are known. In light of the abundance of available solutions, the patent documents DE 21 17 060; DE 25 29 591; DE 32 20 995; DE 35 18 362; U.S. Pat. No. 2,904,575; and DE 41 00 632 are cited here as representative of such solutions.
A process is known from WO 97/31707 in which a trimmer heater is arranged upstream of a cooperating gas/gas heat exchanger, which reduces the temperature difference of the heat exchanger and thus increases its surface area. The aim of the present invention is to avoid such an increase of the surface area. WO 97/31707, furthermore, also describes a technology in connection with which the heat exchanger is integrated in the reactor. This necessarily leads to the necessity for having an additional heat exchanger because the conditions of the reaction are otherwise unstable.
In DE 40 28 750 of the present applicant firm, a furnace (=6 in said patent document) is employed for reaching the required reaction temperature on the inlet of the adiabatic reactor. Said furnace also serves as an indirect heater for a re-boiler (=11 in said patent document) employed in the distillation stage. Only a small portion of the reaction heat of the reactor is used there for heating the reaction inlet gases. The re-boiler only serves there the purpose of propelling the subsequent distillation.
DE 40 04 862 of the present applicant firm describes an additional heat exchanger which, however, is employed there only as a heat exchanger in the start-up phase. The use of said heat exchanger, however, does not result in any reduction of the remaining heat exchanger surface areas.
It is stated in DE 41 00 632 that it is advantageous to operate a number of synthesis stages with reactors, to condense the methanol produced by said reactors, to separate the methanol from the synthesis gas, and to extract the methanol downstream of each individual reactor in the form of a separate methanol product stream. In the cooling stage for cooling the methanol synthesis gas mixture exiting from the reactors, it is economical to use the heat to be dissipated at least partly for the purpose of preheating the inlet streams admitted into the individual reactors by means of gas/gas heat exchangers.
However, in the course of many years of operation it has been found to be a drawback that catalyst material normally used in the methanol reactors is subject to changes occurring in the course of time with respect to its activity in such a way that increasingly higher reactor temperatures are required in order to obtain an optimal yield, whereby, however, such higher reactor temperatures can no longer be reached after some time in the zones downstream of the gas inlet areas. Because of the reduced reaction turnover, both less methanol and also less steam are produced, which is normally collected in the cooling phase of the methanol reactors in the form of a coupled product.
The problem of the invention is to overcome the aforementioned drawbacks and in particular to provide the possibility for saving heat exchanger surface area and/or to increase the yield while particularly taking into account the change occurring in the catalyst activity.
In a process of the type specified above, said problem according to the invention is solved with the help of the characterizing features of claim 1. Provision is made in this connection that in at least one synthesis stage, the gas mixture intended for use in a methanol reactor filled with catalyst is directly heated in an additional trimmer heater before it is added to said methanol reactor.
Further developments of the invention comprise the following features, among other things:
The temperature of the gas mixture intended for use in the catalyst-filled reactors is measured after it has been heated by the additional trimmer heater;
the feed of heat into the additional trimmer heater can be controlled; and
the additional trimmer heater is heated with a foreign medium, for example steam.
The principal advantage of such a procedure lies in that the gas/gas heat exchanger cooling the stream exiting from the reactor against the incoming stream can be designed with distinctly smaller dimensions. Furthermore, it was surprisingly found that with diminishing catalyst activity it is always possible to obtain an optimal yield by after-controlling the reactor inlet temperature.
Since the heat transfer properties of a, for example steam-heated trimmer heater are by multiple times superior to those of a gas/gas heat exchanger, and because the required safety reserve can be solely attributed to the trimmer heater, the result xe2x80x9cunder the bottom linexe2x80x9d even includes savings of investment costs. Furthermore, it was found that the amount of steam used over the useful life of the catalyst is smaller than the loss of coupling product in case of loss due to reduced reaction turnover. The amount of steam generated thus increases xe2x80x9cunder the bottom linexe2x80x9d in spite of the additional steam requirement. Furthermore, it was found that the start-up period of such a production plant can be substantially reduced if trimmer heaters can be used as starter heaters.
The problem stated above is solved with a system of the type specified above in that provision is made in at least one synthesis stage for a trimmer heater for heating the gas mixture that is directly acting on the methanol reactor.
Further developments of the system are specified in the system claims, whereby the advantages that have to be associated with said systems correspond with the advantages offered by the corresponding method.